Sinus tach or SVT? 4 clues to tell the difference
Instead of "rules" you have been taught to distinguish sinus tach and SVT, follow these assessment and ECG interpretation tips
Rescue One is called to a car versus tree crash. The driver is a 60-year-old man who is agitated and has slurred speech. His pulse rate is 160 beats per minute. He is uncooperative, but no injuries can be found on survey. He admits to drinking "a lot" of alcohol. His BP is 150/84. This is his ECG:
Does this patient have sinus tachycardia or does the ECG show supraventricular tachycardia? For this discussion, SVT will refer to atrial tachycardia, AVNRT, or AVRT.
In addition to the rhythm, there is also a left anterior fascicular block (left anterior hemiblock), which is causing the left axis deviation.
Were you taught any of the following?
- SVT is always more symptomatic than sinus tach.
- Sinus tachycardia has a rate of 100 to 150 beats per minute and SVT has a rate of 151 to 250 beats per minute.
- With sinus tach, the P waves and T waves are separate. With SVT, they are together.
These "rules," which you may have been taught, are not correct.
- The symptoms caused by tachycardia vary according to the rate, the other medical conditions the patient has and other factors. For instance, someone with a high fever and dehydration due to the flu will be very symptomatic. A person who has been running will feel better than the person with the flu, even if they have the same heart rate.
- Sinus tachycardia can occur at rates greater than 150 bpm. Just hop on a treadmill and keep going until your rate is 150. Can you push it to a faster rate? Of course you can. Supraventricular tachycardia can occur at rates under 150 bpm.
- Whether the P waves and T waves occur on the ECG separately or together depends on the rate, the intervals and the lead you are looking at. So, a sinus tachycardia at 150 and an SVT at a rate of 150 can have the same likelihood of P-on-T.
4 tips to differentiate sinus tach from SVT
So, how do we differentiate sinus tachycardia from SVT? Here are four tips that will help.
1. Look at the patient’s presenting complaint.
This one assessment will usually identify the source of the tachycardia. For example, a firefighter has just come out of a house fire. He was inside for 30 minutes and is hot and exhausted. His heart rate is 160 bpm. Is this sinus tachycardia or SVT?
This is sinus tach because we can easily explain the reason for the rapid heart rate. In addition, when we sit him down, remove his bunker coat and let him cool off and drink water, the rate gradually returns to normal.
A woman calls 911 because her heart is racing. She tells you that she was reading a story to her grandchild when the symptoms started suddenly. Her heart rate is 160 bpm. Her skin is warm and dry, and she is not pale or cyanotic. Her breathing is normal. She has no chest pain. Is this sinus tach or SVT?
This is SVT because there is no obvious reason for sinus tach and the onset was sudden.
2. Look for the beginning and/or the end of the tachycardia.
Sinus tachycardia warms up and cools down. That is, when the body needs a faster rate, chemical signals are sent to the SA node and it speeds up gradually. Then, when the need for the faster rate is past, the heart rate slows gradually. This speeding up and slowing down may take a few seconds or many minutes, but it is not sudden.
SVT is often called paroxysmal supraventricular tachycardia (PSVT) because it begins suddenly and ends suddenly. PSVT is due to a mechanism called re-entry. A premature beat gets caught in a loop, repeating itself over and over. The impulse is caught in a circular pathway that has a slow area within it. The impulse is delayed in the slow area, then finds when it exits that the heart has recovered from the last beat. That same impulse is able to re-enter the heart and cause another beat. This occurs over and over until something breaks the cycle.
If the termination of the tachycardia is also abrupt, we know it is PSVT. Seeing the beginning or end of the tachycardia is a gift that makes diagnosis easy. If we aren’t lucky enough to see the beginning or end, then the faster the rate, the more likely it is SVT.
3. Look for rate variability.
SVT starts suddenly then stays at about the same rate until it ends. Sinus rhythms frequently change rates in response to the messages received from the nervous system. So, when we encounter a narrow-complex tachycardia that has gradual speeding up or slowing down over seconds to minutes, it is a sinus tachycardia. If the rate changes abruptly from a slow rate to a fast rate and the fast rate is the same rate from beginning to end, we are looking at PSVT. Be sure to run long enough rhythm strips to appreciate rate changes or to catch the onset or offset of tachycardia.
4. Look carefully for P waves.
Sinus tach and most SVTs have only one P wave for each QRS complex. They may or may not be buried in the preceding T waves. But there are other supra-ventricular tachycardias that have more than one P wave for each QRS or no P waves.
Atrial fibrillation has no P waves. You may see some waves that look like P waves, but they will not march out in front of the QRS complexes in a regular rhythm. And, of course, atrial fibrillation is irregularly irregular.
New-onset atrial flutter most often presents with 2:1 conduction. This can be mistaken for sinus tachycardia, if all flutter waves are not seen. Look for flutter waves “hiding” behind the QRS complexes.
Rescue One vehicle crash patient
The Rescue One crew gave the inebriated patient fluids and diltiazem. Over the next hour, his heart rate gradually slowed. Sinus tachycardia is common in acute alcohol intoxication, due to catecholamine release and sometimes dehydration. The patient was also anxious and agitated on arrival to the ED. He became cooperative and jovial after the first hour in the ED. Further evaluation found no injuries secondary to the car crash. We do not know if he had a cardiology follow-up for the left anterior fascicular block.
Before deciding which treatment route to pursue, perform a good history and physical and watch the ECG for onset/offset of the tachycardia, rate variability and P waves. Treatment for sinus tachycardia is almost always aimed at the cause — anxiety, pain, hypovolemia, hypoxia, drugs, fever, etc.
Treatment for re-entrant SVTs may include a Valsalva maneuver initially. The first line drug for PSVT is adenosine. Other drugs may also be useful, including beta blockers and calcium channel blockers. Unstable patients with PSVT should be electrically cardioverted.